The present invention relates to a cathode ray tube; and more particularly, to a cathode ray tube capable of effectively preventing the failure thereof by way of optimizing the location of a mold match line.
As well known, a glass bulb in a cathode ray tube (CRT) used in a TV set or a computer monitor basically includes a panel for displaying picture images, a conical funnel sealed to the back of the panel and a cylindrical neck integrally connected to an apex portion of the conical funnel. The panel, the funnel and the neck are made of glass, wherein particularly the panel and the funnel are formed of predetermined dimensions and shapes by press forming a glass gob.
Referring to FIG. 1, there is illustrated a cross sectional view of a conventional glass bulb 10. A panel 20 of the glass bulb 10 is provided with a face portion 21 whose inner surface is covered with an array of dots of fluorescent material (not shown) to display picture images; a skirt portion 23 extending backward from a perimeter of the face portion 21 and having a seal edge 22 on its back edge; and a blend round portion (or corner portion) 24 integrally joining the face portion 21 to the skirt portion 23. A funnel 30 of the glass bulb 10 can be divided into a body portion 32, i.e., a fore part thereof, having a seal edge 31 connected to the seal edge 22 of the skirt portion 23; and a yoke portion 33, i.e., a back part thereof, extending backward from the body portion 32. And a neck 40 of the glass bulb 10 is connected to the yoke portion 33 of the funnel 30. A tube axis 11 passes through the center of the face portion 21 and coincides with an axis of the neck 40. Placed by way of the so-called xe2x80x9cshrinkage fitxe2x80x9d scheme around the outer periphery of the skirt portion 23 is a metallic implosion-proof band 50, which strengthens the bulb 10 against tensile stress induced in the blend round portion 24 and the skirt portion 23 by evacuating the inner space of the bulb 10, so that fragments of the glass can be prevented from flying away when the panel 20 is broken or exploded.
Referring to FIG. 2, there is illustrated a schematic cross-sectional view of a mold set 60 for forming the panel 20. The mold set 60 is provided with a bottom mold 62 in which a cavity 61 is formed; a middle mold (or shell) 63, for forming the skirt portion 23 and the seal edge 22, which is fitted on top of the bottom mold 62; and an upper mold 64 (or plunger) which presses a glass gob loaded in the cavity 61 of the bottom mold 62 to form the panel 20. The upper mold 64 is connected to a press ram 65, so that it can be lifted or lowered by the ram 65 so as to press the glass gob loaded in the cavity 61 of the bottom mold 62 to form the panel 20. There exists a parting line 66 between the bottom mold 62 and the middle mold 63. Therefore, when the panel 20 is formed in the mold set 60 as shown in FIG. 1, a mold match line 25, which is a flash made by the parting line 66, is formed on the outer periphery of the skirt portion 23 near the face portion 21. The peripheral length of the mold match line 25 represents the maximum peripheral length of the panel 20. And, in general, the position of the parting line 66 and thus the position of the mold match line 25 are set near the face portion 21 rather than the seal edge 22 in order to ease the extraction of the molded panel 20 from the bottom mold 62.
Referring to FIG. 3, there is illustrated a schematic cross sectional view of the panel 20 of FIG. 1. Formed on the outer periphery of the skirt portion 23 is a first tapered surface 26, which extends from the mold match line 25 toward the seal edge 22 with an inward slant of a first slant angle xcex81 with respect to the tube axis 11. And the first slant angle xcex81 is set less than 1.5xc2x0 in order to prevent the band 50 from slipping. There is formed a second tapered surface 27 between the first tapered surface 26 and the seal edge 22. The second tapered surface 27 has a second slant angle xcex82 ranging from, e.g., 3xc2x0 to 4xc2x0. The border where the first tapered surface 26 meets with the second tapered surface 27 is referred to as a break line 28, which is positioned away from a lower end of the implosion-proof band 50.
When the explosion-proof band 50 is installed around the outer periphery of the skirt portion 23 of the glass bulb 10 after the glass bulb being evacuated, the implosion-proof band 50 expanded by heating is pushed from the side of the face portion 21, and then is fitted around the skirt portion 23. At this time, the mold match line 25 is brought into contact with the implosion-proof band 50, which may incur scratches or/and cracks in the mold match line 25. Therefore, in a case where the mold match line 25 is formed near the face portion 23, an area of the inner surface of the implosion-band 50 sweeping or scrubbing the mold match line 25 is comparatively greater and thus, the frequency of the contacts between the mold match line 25 and the implosion-proof band 50 increases. Consequently, a greater number of cracks and scratches are formed in the mold match line 25. Further, additional cracks are developed in the mold match line 25 by a compressive stress generated by the shrinkage of the expanded implosion-proof band 50.
Such cracks formed in the mold match line 25 of the skirt portion 23 are one of the major causes for rendering the breakage or failure of the cathode ray tube occurring when the cathode ray tube is subject to a heat treatment in an annealing lehr for removing remaining stress therefrom, resulting in an increase of the production cost and the deterioration of production yield.
It is, therefore, an object of the present invention to provide a panel for use in a cathode ray tube (CRT) capable of effectively preventing the failure of the CRT, such as implosion or breakage, during a heat treatment process of the CRT by optimizing the location of the mold match line.
It has been found by the inventors of the present invention that the cracks and scratches can be effectively prevented from forming in the mold match line by locating the mold match line such that the inner surface area of the implosion-proof band sweeping the mold match line is comparatively less and the mold match line is subject to the comparatively less compressive force produced by the implosion-proof band.
In accordance with a preferred embodiment of the present invention, there is provided a panel for use in a cathode ray tube, including: a face portion for displaying picture images, whose mean outer radius of curvature is equal to or greater than 10,000 mm; a skirt portion extending from a periphery of the face portion and having a seal edge sealed to a funnel; and a blend round portion connecting the face portion and the skirt portion, wherein a mold match line is formed on the skirt portion in a manner that a mold match line height H1 satisfies a following equation: 0 less than H1xe2x89xa6Hxc3x970.47 where an overall height H is a distance between a first plane passing through the seal edge and a second plane passing through a center of the face portion, and the mold match line height H1 is a distance between the first plane and a third plane passing through the mold match line.
In accordance with another preferred embodiment of the present invention, there is provided a panel for use in a cathode ray tube, including: a face portion for displaying picture images, whose mean outer radius of curvature is less than 10,000 mm; a skirt portion extending from a periphery of the face portion and having a seal edge sealed to a funnel; and a blend round portion connecting the face portion and the skirt portion, wherein a mold match line is formed on the skirt portion in a manner that a mold match line height H1 satisfies a following equation: 0 less than H1xe2x89xa6Hxc3x970.37 where an overall height H is a distance between a first plane passing through the seal edge and a second plane passing through a center of the face portion, and the mold match line height H1 is a distance between the first plane and a third plane passing through the mold match line.